1. Field of the Invention
The present invention relates to a method of coal liquefaction.
2. Description of the Prior Art
FIG. 4 shows a schematic flowchart of a conventional method of coal liquefaction. According to the conventional method, a pulverized coal and a coal liquefied oil (solvent) obtained from the distillation step described later are charged into the slurry tank 2, where they are mixed together under agitation to prepare a coal slurry. The coal slurry is pressurized and mixed with a gas (recycle hydrogen gas) consisting mainly of hydrogen which was separated in the gas purifying step described later, and they are introduced into the heating furnace 3. The coal slurry introduced into the heating furnace 3 is pressurized to a pressure of 100 atm or more and heated to a temperature of 400.degree. C. or more, and feed into the coal liquefaction reactor 4. The coal liquefaction reactor 4 conducts a liquefaction reaction under a hydrogen positive pressure and at an elevated temperature.
The product of the liquefaction reaction leaving the reactor 4 enters into the gas separator 6 where the product is separated to a gas and a liquefied slurry containing liquefied oil and non-liquefied matter.
The liquefied slurry contains a substantial amount of ash and non-liquefied matter consisting mainly of unreacted organic residue. Since such non-liquefied matter causes trouble in the succeeding treatment such as distillation, the liquefied slurry is sent to the filter 30 to separate the non-liquefied matter. The liquefied solution free of non-liquefied matter is sent to the distillation unit 8 to be fractionated into light oil and fuel oil, and to recover the liquefied oil. A part of the liquefied oil is charged to the slurry tank 2 as the solvent for preparing the coal slurry. The filter cake separated by the filter 30 is sent to the hydrogen manufacturing facility 31 as the raw material for hydrogen production, and is gasified there.
On the other hand, the gas separated in the gas separator 6 is sent to the gas purification unit 7 for purification. Since the gas consists mainly of hydrogen, the gas is recycled and is added to the coal slurry which is fed to the liquefaction reactor 4. However, the hydrogen that is recycled is not sufficient to carry out the liquefaction reaction, and hydrogen obtained by gasification of the filtrate discharged from the hydrogen manufacturing facility 31 is added to the coal slurry. The hydrogen manufacturing facility 31 consists of many treatment stages including the gasification stage where the filtrate is completely decomposed under the presence of oxygen, the purification stage for purifying the generated decomposed gas, the hydrogen-enriching stage where the CO gas in the generated gas is shift-reacted to yield a hydrogen-rich gas, the gas cooling stage, and the stage for CO.sub.2 removal from the gas, using alkali. In this manner, the hydrogen manufacturing facility is very complex.
According to the above-described method, the liquefaction reaction has to use hydrogen which is produced in an extremely complex hydrogen manufacturing facility 31. Since the hydrogen manufacturing facility 31 is very complex, it is expensive (as high as nearly 40% of the total investment of the liquefaction plant, in some cases), as well as involving a high operating cost. Therefore, the share of hydrogen manufacturing cost to the total coal liquefaction product cost becomes very high.